Process for improving the adhesion of a lubricant to the surface of an aluminum strip

ABSTRACT

A process for improving the adhesion of a lubricant to, and the distribution on, the surface of a strip of aluminum or aluminum alloy which is cold rolled between two work rolls, said cold rolled strip being coated with lubricant for the purpose of subsequent shape-forming, is such that the surface of at least one work roll exhibits a roughness pattern ( 10 ), which is comprised of at least two line patterns ( 14,18 ) that cross each other, whereby each of these line patterns is characterised by a plurality of parallel rills ( 12,16 ).

BACKGROUND OF THE INVENTION

[0001] The invention relates to a process for improving the adhesion of a lubricant to, and the distribution on, the surface of a strip of aluminum or aluminum alloy which is cold rolled between two work rolls, whereby the cold rolled strip is coated with lubricant for subsequent shape-forming.

[0002] In the automotive industry strips and sheets of aluminum alloys are employed for manufacturing car body parts, whereby the sheets are formed e.g. by deep-drawing or stretch-drawing. Lubricants have to be employed for carrying out this deep drawing or other shaping process. By pre-coating the strips or sheets in the rolling mill not only is the forming operation in the overall process simplified, but also optimal protection provided during transportation.

[0003] A conventional procedure today is such that the cold rolled strip is coated by the strip manufacturer with oil or water-soluble dry lubricant and delivered to the sheet forming customer as pre-treated strip.

[0004] The lubricants exhibit relatively poor adhesion and distribution on the surface of aluminum strip which has been manufactured in the conventional manner using work rolls that have a roll grind corresponding to a mill-finish surface texture.

[0005] The adhesion and distribution properties of dry lubricants on the strip surface can be improved significantly by providing the surface of the strip with a roughness pattern. In that case the roughening of the sheet surface takes place via the corresponding roughened surface of the work rolls by transferring the roughness pattern during the cold rolling of the strip.

[0006] Today, the roughness patterns employed to improve the adhesion and distribution of a dry lubricant to the surface of an aluminum strip are produced on work rolls which have been roughened by electrical discharge texturing (EDT), by electron beam texturing (EBT) or by the PRETEX method. All of these roughening pro-cesses result in a surface texture with irregularly distributed, “closed pockets” of lubricant.

[0007] The disadvantage of these known roughness patterns with a statistical distribution of peaks and troughs which lead to closed “pockets” of lubricant is that such a roughness pattern can be transferred to the strip surface only with a small reduction in thickness of 5 to 12% in the last cold rolling pass, while the normal cold rolling passes effect a 30 to 60% reduction in thickness. In practice, this means an additional cold rolling pass and related higher production costs.

[0008] The object of the present invention is to provide a process of the kind mentioned at the outset which permits the strip to be cold rolled with the normal reduction in thickness.

SUMMARY OF THE INVENTOIN

[0009] The foregoing object is achieved by way of the invention in that the surface of at least one work roll exhibits a roughness pattern, which is comprised of at least two line patterns that cross each other, whereby each of these line patterns is character-ised by a plurality of parallel rills.

[0010] For certain forming processes it would basically suffice if only one side of the strip is coated with lubricant. For that case it would be adequate for only one of the two work rolls to feature the roughness pattern according to the invention. In practice, however, the roughness pattern is usefully applied to both work rolls.

[0011] Basically it is conceivable to have a roughness pattern made up of three or more line patterns each with parallel rills. However, the roughness pattern preferred in practice is comprised of two line patterns that cut across each other.

[0012] Particularly good results can be achieved if two line patterns are arranged at 30 to 60° to the rolling direction, whereby the line patterns preferably cut each other at an angle of 90°.

[0013] The roughness patterns are preferably chosen such that the distance between neighbouring rills and the depth of the rills lie between 0.2 and 2.5 am, preferably between 0.4 and 1.8 μm. The distance and the depth of the rills are also known as average roughness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Further advantages, features and details of the invention are revealed in the following description of a preferred exemplified embodiment of the invention and with the aid of the drawing which shows schematically in

[0015]FIG. 1 illustrates a greatly magnified section of the surface of a work roll.

DETAILED DESCRIPTION

[0016] Shown in FIG. 1 is roughness pattern 10 on the surface of a work roll—not shown here—for cold rolling aluminum, said roughness pattern 10 comprising a plurality of parallel first rills 12 which form a first line pattern 14 and a second line pattern 18 comprised of a plurality of parallel second rills 16. Both line patterns 14, 18 cut across each other at an angle γ which in the present example is 900. The first line pattern makes an angle α with the rolling direction x, the second line pattern 18 an angle β. In the present example both angles are 45°. The rolling direction corresponds with the circumference of the work rolls.

[0017] The roughness of the roughness pattern 10 is a result of the average distance a between neighbouring rills. The distance a corresponds essentially also to the depth of the individual rills.

[0018] In practice the roughness pattern 10 is created on the surface of a work roll in a conventional manner by grinding the roll in the two given grinding directions. The resultant average roughness of the rolls is in the range 0.4 to 1.8 μm.

EXAMPLE

[0019] A continuously cast slab of alloy AA 6181a was hot rolled in a conventional manner to a thickness of 4 mm. The subsequent cold rolling to a final thickness of 1 mm took place using two work rolls of differently ground surfaces. The reduction in thickness of the strip in the last cold rolling pass was 45%.

[0020] The surface of both strips was then sprayed with the dry lubricant ALUB ZX. After evaporation of the solvent, the distribution of the dry lubricant on the surface was assessed visually. The results of these investigations are summarised in the following table. Roughness pattern on the Distribution of lubricant on the work rolls strip surface According to the invention uniform Mill finish locally different

[0021] The roughness pattern of the work rolls roughened according to the invention corresponded with that in FIG. 1. The line patterns made an angle of 45° with the rolling direction and lay therefore at 90° to each other. The average roughness of the surface of the work rolls lay in the region of 0.4-1.8 μm.

[0022] The roughness pattern taken here for comparison purposes—designated as “mill finish”—corresponds to the conventional roll grind employed today with work rolls. The rolls are only ground in their circumferential direction i.e. the roll grind is made up of a plurality of parallel rills distributed over the circumference of the work rolls. The average roughness of the roll surface lay in the region of 0.4-1.8 μm.

[0023] The example shows clearly the superiority of surface texturing by cross-grinding rolls over the simple roll grinding which results in a mill-finish surface quality.

[0024] By uniform distribution of lubricant on the surface of the strip the amount of lubricant to be deposited on the surface can be considerably reduced. This leads not only to more uniform distribution but also to a reduction in the removal of lubricant in the forming tool. Apart from a reduction in the amount of cleaning required, there is also a significant reduction in the danger of surface damage by the shaped part as a result of contamination of the forming layer. 

1. A process for improving the adhesion to, and distribution of, a lubricant to the surface of a strip of one of an aluminum and aluminum alloy which is cold rolled between two work rolls, whereby the cold rolled strip is coated with lubricant for subsequent shape-forming, the process comprising providing on the surface of at least one work roll a roughness pattern (10), which is comprised of at least two line patterns (14,18) that cross each other, whereby each of these line patterns is characterised by a plurality of parallel rills (12,16) and rolling the strip with the surface of the at least one work roll.
 2. A process according to claim 1, wherein the roughness pattern (10) is comprised of two line patterns (14,18) that cross each other.
 3. A process according to claim 2, wherein the line patterns (14,18) make an angle (α,β) of 30° to 60° with the rolling direction (x).
 4. A process according to claim 3, characterised in that the line patterns (14,18) cross at an angle (α) of 90°.
 5. A process according to claim 1, wherein the distance (a) between the neighbouring rills (12,16) and the depth of the rills (12,16) lie between 0.2 and 2.5 μm.
 6. A process according to claim 1, wherein the distance (a) between the neighbouring rills (12,16) and the depth of the rills (12,16) lie between 0.4 and 1.8 μm.
 7. A process according to claim 1, wherein the roughness pattern (10) is created on the surface of both work rolls. 